The invention relates to a method for delaying an ultrasound signal consisting of a carrier signal with an envelope, where two partial signals derived from the ultrasound signal are delayed separately. The invention also relates to an apparatus for carrying out the method. The invention is suitable for, among other uses, use in a phased array, annular array or dynamically focused linear array.
In medical ultrasound technology, in particular with phased arrays, linear arrays with electronic focusing, annular arrays, or curved array equipment, delaying received ultrasound signals in different processing channels is a major problem. Each processing channel processes ultrasound signals of one (or more) ultrasonic transducer(s) of the respective array. A plurality of such ultrasound signals, in a frequency range between 3 and 5 MHz at a bandwidth of about 1 MHz, must be delayed in small time increments to a total on the order of 0 to 10 .mu.sec or even more. The delay is chosen so that the different transit times of the ultrasonic echo from a target point to the ultrasonic transducer elements are compensated.
This has been done for example by applying the surface acoustic wave technique (SAW), which in itself is known from video technology. There, however, frequencies between 15 and about 40 MHz are processed, so that applying this technology in the ultrasonic range requires upward mixing of the ultrasound signals into this higher frequency range. The disadvantages here are (a) that expensive and costly components are needed and (b) that because of the high attenuation of the signals, problems occur in the switching of filters in this frequency range.
From video technology comes also another technique using the charge coupled device (CCD). CCD components, however, are also expensive and difficult to drive. Besides, interference appears in the desired signal, and this is difficult to filter out.
Digital delay methods which also can be employed in ultrasonic arrays all have the disadvantage that, in a frequency range up to 5 MHz, the received signals must be sampled at about 15 to 20 MHz. Analog/digital converters which operate in this range are expensive and inaccurate.
LC delay lines are commonly used for delaying ultrasound signals. At high frequencies of e.g. 5 MHz they, too, have the disadvantage that relatively long delay times can be realized only at high expense.
Additional solutions for delaying an ultrasound signal are described in DE-PS No. 28 54 134 and in a publication by G. F. Manes et al, "A new delay technique with application to ultrasound phased array imaging systems" Ultrasonics, September 1979, p. 225-229. There the received ultrasound signal, which lies in the range of from 3 to 5 MHz, is brought to a lower or higher frequency level by a heterodyning technique. For higher frequency levels, delaying by means of the SAW technique is possible. For lower frequency levels, LC delay lines are used. The oscillator signal with which the ultrasound signal is mixed is phase-adjusted for each ultrasound channel in these two solutions. This adjustment is chosen such that the phases of the carriers of the resulting mixed signals are the same in all channels. By alteration of the phase an approximate fine delay of the ultrasonic signal is obtained. For longer delay times, the mixed signal of each channel is then coarsely delayed in a second step by means of analog delay lines in steps which correspond to an integral multiple of the period T of the carrier oscillation of the ultrasound signal. The phase position of the individual mixed signals remains constant so that the signals are coherent. One disadvantage of this method is the component expense involved in the mixing of the signals; a second is that coherence of the phases can be produced exactly only for a single frequency. Since the received ultrasound signal has a certain bandwidth, imprecisions arise. The wider the bandwidth of the received ultrasound signal, the less precisely will this method operate.
In the technique described in the Manes article at FIG. 1, the mixing takes place, not to a lower frequency, but to zero frequency. It involves, therefore, the so-called baseband method. Consequently quadrature processing of the signal is required. The quadrature processing employed there involves two separate delay channels with a plurality of components, which makes the solution costly.
Delay components with widely adjustable delay times are either not available at all for certain frequency ranges or are expensive.
Objects of the invention are to delay ultrasound signals using inexpensive components and over a relatively wide bandwidth.